Light-sensitive black-and-white as well as color photographic silver halide materials comprising silver halide emulsion layers having negative image type tabular silver halide emulsion crystals or grains have become more and more important during the last decade. Tabular silver halide grains are meanwhile well-known as crystals possessing two parallel faces with a ratio between a diameter of a circle having the same area as these faces, and the thickness, being the distance between the two major faces, equal to at least 2. Tabular grains are known in the photographic art for quite some time. As early as 1961 Berry et al. described the preparation and growth of tabular silver bromoiodide grains in Photographic Science and Engineering, Vol 5, No. 6. A discussion of tabular grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, p. 66-72. Early patent literature includes Bogg, U.S. Pat. No. 4,063,951, Lewis U.S. Pat. No. 4,067,739 and Maternaghan U.S. Pat. Nos. 4,150,994; 4,184,877 and 4,184,878. However the tabular grains described therein cannot be regarded as showing a high diameter to thickness ratio, commonly termed aspect ratio. In a number of U.S.-Patent Applications filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are described as in U.S. Pat. Nos. 4,434,226; 4,439,520; 4,425,425; 4,425,426 and in Research Disclosure, Volume 225, January 1983, Item 22534.
For radiographic applications the main photographic advantages of tabular grains compared to normal globular grains are a high covering power at high forehardening levels, a high developability and higher sharpness, especially in double side coated spectrally sensitized materials. The thinner the tabular grains the greater these advantages.
In the references on tabular grains cited above especially silver bromide or silver bromoiodide emulsions having a high sensitivity are disclosed whereas the use of e.g. emulsions with tabular grains rich in silver chloride was considered to be disadvantageous with respect to sensitivity. For emulsions with crystals rich in silver chloride, applications in the field of less sensitive materials as e.g. graphic arts materials, duplicating materials, radiographic hardcopy materials, diffusion transfer reversal materials and black-and-white or colour print materials are well-known. The advantages of said emulsions with crystals rich in chloride regarding higher development and fixing rates, are highly appreciated. Indeed as nowadays the tendency is present to get materials processed in shorter processing times, it is highly appreciated to combine said advantages with a high sensitivity for application in high-sensitive materials, an object which can be realized as has been described in EP-A 0 678 772.
Just as in applications mentioned hereinbefore in the field of industrial radiography, especially for non-destructive testing applications, any time saving measure is welcome: after exposure with direct-rontgen rays, industrial non-destructive testing film is automatically processed in a cycle, varying from 8 to 12 minutes, wherein the tendency is to reduce the processing time to a maximum of 5 minutes. One method to reach that goal has been described in U.S. Pat. No. 5,397,687 wherein cubic silver halide crystals rich in chloride are used, permitting further a decreased fixing time for the non-developed silver halide crystals rich in silver chloride in a still acceptable short time. Rapid processing of silver halide crystals rich in chloride however leads to high contrast and a higher noise level (more granularity).
Otherwise silver cubic bromoiodide grains, although having a slower development rate, are used preferably in NDT-applications for the following reason. In order to achieve high film speed, which is an indispensible asset especially for direct-rontgen applications, efficient absorption of the exposure radiation is a prime condition. It has been shown empirically that for X-rays the mass absorption coefficient is proportional to a power of the atomic number Z as has been described in the "Encyclopaedic Dictionary of Physics"vol. 7, p. 787, eq. 10, Ed. J. Thewlis, Pergamom Press, Oxford 1957. This strongly disfavours the use of chloride (Z=17) compared to bromide (Z=35) or iodide (Z=54). As a consequence bromide and iodide ions released in the developer further inhibit development of the remaining developable silver halide crystals, so that the regeneration capacity (replenishment) of the developer should be increased resulting in consumption of higher amounts of chemicals, a higher cost and more environmental load.
One method to reduce processing time and consumption of chemicals consists in lowering coated amounts of silver. A reduction of sensitivity for direct-Rongten rays normally leads to a lowering in contrast, which is in favour of image quality (especially graininess) but makes maximum density decrease to an unacceptable level. When moreover only use can be made of radiation sources for X-rays having a lower energy output (exposure energies of about 100 kVp instead of the normally used 220 kVp) a higher exposure contrast further leads to even higher contrasts and reduced speed. Although said reduced speed can be compensated in industrial radiographic exposure techniques by application of intensifying screens in contact with industrial non-destructive test film materials, thereby taking profit of the combined effect of direct-Rontgen exposure and exposure by light emitted from light-emitting phosphors present in the intensifying screens, the problem of too high contrasts remains. Moreover as a consequence of the presence of huge amounts of coated silver there is a tendency to sludge in the processing solutions as a consequence of a substantial contribution of physical development.